This invention relates generally to enabling aircraft wireless links to achieve the high data rates provided by MIMO (Multiple Input-Multiple Output) communications over longer distances than usually provided with limited size aircrafts. The invention described herein employs a large size ground array MIMO relay.
The wireless link between aircraft at elevated altitudes is typically free of electromagnetic reflections (termed free-space communications). MIMO links rely on the ability to distinguish between the transmitting array elements by the receiving array. Without electromagnetic scattering, the transmitting array element spacing must be resolved by the receiving array free-space beamwidth. The beamwidth of the receiving array is limited by its size, which is limited by the size of the aircraft. For typical size aircraft and typical radio frequencies, link distances greater than 1 km do not provide the MIMO data rates expected for spectrum efficiency. It is desirable to provide MIMO capacity for aircraft links longer than 100 km. A way of overcoming the limitations imposed by aircraft size is desired.
The distance limitation for a wireless aircraft to aircraft MIMO link is highlighted in the reference, M. J. Gans, “Aircraft Free-Space MIMO Communications”, Asilomar Conference on Signals and Computers, Nov. 1-4, 2009—Paper MP7a-1.
FIG. 1 shows the channel capacity as a function of distance and frequency for two example F-35 airplanes for two arrays with 12 elements each. The graph shows for 10 GHz, the dependence of channel capacity on longitudinal separation for the twelve element array case with 100 m lateral and height separations of the parallel flight paths. Note that the lateral and height separations prevent the airplanes from approaching nearer than 140 meters. The channel capacity actually exceeds the ergodic Rayleigh capacity mostly out to a longitudinal separation of 700 m. The rapid decrease in channel capacity for link lengths greater than 1 km is due to the insufficient resolving power of the array because of the limited aircraft size.
One method of increasing the possible communications link length using MIMO on a fixed aircraft size is to increase the radio frequency. The beamwidth of a fixed width array is inversely proportional to the radio frequency. A much narrower beamwidth can allow transmitting antenna element resolution by the receiving array at larger distances. However, using a many times higher frequency can be subject to high propagation loss and expensive hardware.
In U.S. Pat. No. 6,097,771 to G. J. Foschini, “Wireless Communications System Having a Layered Space-Time Architecture Employing Multi-Element Antennas” it is taught that wireless link data rates can be increased roughly in proportion to the number of antenna elements at the link terminals, without increasing the total transmitted power or bandwidth. This allows for enormous data rates, e.g., hundreds of bits per second per Hz for a system employing 30 antennas at both transmitter and receiver and experiencing an average signal-to-noise ratio of 24 dB in a so-called Rayleigh fading environment. This is the high data rate MIMO capacity sought after in the free-space aircraft environment, referred to above.